Last year (Sept. 1976 - Dec. 1977) we reported that single exposures of 50 mR X-rays consistently resulted in no abnormal levels of marrow nucleated red cells (RBCp) contents in terms of RBCp%, RBCp/whole femur or RBCp/skeleton when animals were five through 30 weeks post X-irradiation. Animals exposed to 150 mR or higher (550, 770 or 1500mR), on the other hand invariably demonstrated marrow erythroid alterations. In the present report (Dec. 1977 - present) it was found that "subthreshold" dosages (25 or 50 mR) when employed two or more times within a seven day period consistently yielded alterative normoblastic changes when the summated dosages exceeded 100 mR. The findings suggest our "subthreshold" dosages, so-called because no measurable effects could be observed following single exposures, could in fact, elicit changes which are too small to be measured. When the marrow is irradiated repeatedly, these subliminal effects are summated and measurable marrow effects are then detected. This phenomenon will be further studied during the next report period using intervals between acute subthreshold dosages of weeks to months. The goal will be to find an interval sufficiently prolonged that the subthreshold effects can no longer be summated to a level detectable by our animal model. Other studies to be included in the next year will involve further work on the low-dose X-ray effects on the splenic colonies of subralethally irradiated mice, a study of the possible pathologic effects triggered by phlebotomy in rats previously exposed to low-dose X-rays, as well as a study on the effects of chronic exposure with low dose-rate Cs-137 gamma radiation.